from Haskell et al.:
An observation dating from at least Kiparsky (1982) is that whereas rats-eater is conspicuously ill-formed, mice-eater is not. A major difference between the two, of course, is that rats is the rule-governed plural of rat, whereas mice is an exception.

with a survey of some earlier literature: abstracts committee and other cases that might involve avoidance of ambiguity; cases where N1 is an irregular plural (mice-eater or mouse-eater); cases where N1 is name of a sports team (Yankees fan or Yankee fan); and a large collection of other cases where N1 is plural presumably to mirror the semantics (Movies Update as an alternative to Movie Update)

final note: “though I’ve followed the literature in talking about N+N compounds like job market as having a singular first element, I’m inclined to say that the first element in such cases is not actually a singular form, but is an unmarked N stem (which of course is phonologically identical to the singular form). This is true whether the first N is mass or count; a count N as modifier is usually interpreted semantically as plural (as in job market).”